1. Field of the Invention
The present invention relates to an image processing method for recording, transmitting, and reproducing monochrome and color density or gray images and dynamic images and, more particularly to an image processing method for performing compression and development of gray image data, and performing coding and decoding of dynamic image information.
2. Related Background Art
In general, when a gray image is recorded on a recording medium, for example, an image read by a scanner is converted into, e.g., density data, and digital data is recorded on a recording medium. However, since recording of all pixel information requires a huge storage capacity, the gray image is divided into recording units coarser than pixels of the scanner, and average values of these recording unit intervals are recorded. Upon reproduction of an image, smoothing and edge sharpening are performed as needed.
Techniques for coding a gray image are roughly classified into a preservation type compression method which preserves a whole information amount, and a non-preservation type compression method which allows an information loss to some extent. As the former method, a DPCM (differential PCM) method, a predictive coding method, a block adaptive type variable length coding method, a bit plane coding method, a hierarchical coding method, and the like are known. As the latter method, a predictive coding method, a transform coding method, a vector quantization method, a block truncation coding method, and the like are known.
Transmission and reproduction of dynamic images have been conventionally performed in the field of television. As a conventional image coding (compression) method, a band compression technique is known. In recent years, along with advances of the digital signal processing techniques and digital communication techniques, data compression based on digital processing is performed.
As is well known, a dynamic image consists of a plurality of frames (e.g., 30 frames per sec). Dynamic image coding techniques are classified into intra-frame coding for independently coding frames, and inter-frame coding for performing processing over a plurality of frames. In the case of a television, since one frame is normally constituted by two fields, the intra-frame coding is further classified into intra-field coding and inter-field coding.
In the intra-frame coding and intra-field coding, a coding technique of a still image can be applied to a dynamic image without modifications. In this case, in order to perform compression more effectively, the inter-frame coding is required. As the inter-frame coding techniques, the following conventional techniques are known:
(i) conditional pixel supplement method PA1 (ii) motion-compensated prediction and background prediction PA1 (iii) inter-frame adaptive prediction as a combination of inter-frame motion-corrected prediction and intra-field prediction PA1 (A) When a dynamic image is recorded or transmitted, a set of points corresponding to image data of 1 in a dynamic image represented by binary data is expressed by one or a plurality of solids in a three-dimensional space which has an X-Y orthogonal coordinate system as an image plane, and a time base perpendicular to the X and Y coordinate axes, a region having the image region as a bottom surface, and a range from an image start time to an image end time as a height is divided into a plurality of rectangular solids, and Rv values proportional to volumes, inside the solid, of the divided rectangular solids are recorded and reproduced; and when the image is reproduced, a surface of the solid crossing the rectangular solids is reproduced on the basis of the Rv values of the plurality of rectangular solids and those of rectangular solids adjacent thereto, and the entire curved surface is reproduced by coupling the reproduced surfaces. PA1 (B) When a dynamic image is recorded or reproduced, a dynamic image represented by multi-value data is expressed by one or a plurality of solids in a four-dimensional Euclidean space which has an X-Y orthogonal coordinate system as an image plane, a time base, and an axis representing a gray level, the four-dimensional space is divided into a plurality of rectangular solids, and Rv values proportional to volumes, inside the solid, of the divided rectangular solids are recorded and reproduced; and when the dynamic image is reproduced, a surface of the solid crossing the rectangular solids is reproduced on the basis of the Rv values of the plurality of rectangular solids and those of rectangular solids adjacent thereto, and the entire curved surface is reproduced by coupling the reproduced surfaces.
The conditional pixel supplement method of item (i) is a technique for extracting and transmitting only data of moving portions of an image. In this technique, an inter-frame difference for each pixel is computed, and a pixel having a difference larger than a predetermined threshold value is output with an address for specifying the pixel.
In the motion-compensated prediction of item (ii), the position of an object of interest in the current frame is predicted by detecting the position and speed of an object of interest in a previous frame. Since the motion-compensated prediction is not effective for a background image which appears after an object moves, a second frame memory is arranged in an encoder to record only a background image separated from the moving object, and the recorded image is utilizes as a background image.
The inter-frame adaptive prediction of item (iii) is a combination of inter-frame motion-compensated prediction and intra-field prediction, and can applied to quick motions.
However, the conventional gray image processing method cannot satisfactorily reproduce a discontinuous change in density in each recording unit since a gray image is divided into recording units coarser than scanner pixels, and average values of the divided recording units are recorded.
The above-mentioned coding techniques require complicated computations, and their transform efficiencies largely depend on the image quality of an original image.
However, the above-mentioned inter-frame coding technique suffers from the following problems.
In the conditional pixel supplement method of item (i), the data amount to be coded undesirably increases with an increase in amount of image motion.
In the motion-compensated prediction of item (ii), the motion detection precision and detectable motion amount are limited. Furthermore, the background prediction requires two frame memories.
The inter-frame adaptive prediction of item (iii) requires a complicated arrangement as compared to the methods of items (i) and (ii).
In recent years, needs for recording, transmission, and compression of dynamic images have been extended to a wide range of media such as video telephones, video conferences, computer graphics, and the like in addition to televisions.